timers should protect against negative values that are recorded

[brharrington]

BasicTimer (and probably others like StatsTimer) do not current prevent negative values from getting recorded. This can happen if say the clock source is not monotonic and there is an ntp adjustment. This will cause an incorrect total time to show up.

Negative values should be ignored, maybe logged so the user will know, though I worry about having logging in the record call if it is used as part of a really tight loop.

[brharrington]

Fixed in #209.